The present invention relates to the field of semiconductor fabrication, and more particularly to an integrated mask and the use of such a mask to fabricate electrodes.
Organic light emitting devices (OLEDs), which make use of thin films that emit light when excited by electric current, are becoming an increasingly popular technology for applications such as flat panel displays. Popular OLED configurations include double heterostructure, single heterostructure, and single layer, and a wide variety of organic materials, some of which are described in U.S. Pat. No. 5,707,745, which is incorporated herein by reference.
Flat panel displays typically include an array of picture elements, or pixels, deposited and patterned on a substrate. Such a pixel array is typically a matrix of rows and columns. In an OLED display, each OLED pixel includes an organic light emitting diode that is situated at the intersection of each column and row line. The first OLED displays, like the first LCD (Liquid Crystal Displays), have typically been addressed as a passive matrix (PM) display. This means that to cause a particular pixel to luminesce, electrical signals are applied to the row and column lines of that particular pixel. The more current that is pumped through each pixel diode, the brighter the pixel appears visually. One method of providing grayscale to the display is to vary the current level of the pixel.
In practice, a voltage is applied to a single row line, and a path for current flow is selectively provided at individual columns. This provides current flow through selected pixels on the single row, thus allowing current to flow causing each pixel in that row line to luminesce at the desired brightness. The next row line is then addressed, and once again, all the pixels on that row line are energized to produce the required brightness. The display continuously scans all the row lines sequentially, typically completing at least 60 scans of the overall display each second. In this way, flicker is not seen since the display is addressed fast enough, for typical observation conditions, that the pixels cannot be seen to be continuously turning on and off. Preferably, the magnitude of current flow through each column can be controlled, such that the brightness of the pixels can be controlled.
For OLEDs from which the light emission is only out of the bottom of the device, that is, only through the substrate side of the device, a transparent anode material such as indium tin oxide (ITO) may be used as the bottom electrode. Since the top electrode of such a device does not need to be transparent, such a top electrode, which is typically a cathode, may be comprised of a thick and reflective metal layer having a high electrical conductivity. In contrast, for transparent or top-emitting OLEDs, a transparent cathode such as disclosed in U.S. Pat. Nos. 5,703,436 and 5,707,745 may be used. As distinct from a transparent or bottom-emitting OLED, a top-emitting OLED is one which may have an opaque and/or reflective substrate, such that light is produced only out of the top of the device and not through the substrate.
The transparent cathode that is used in such a transparent or top-emitting device preferably has optical transmission characteristics such that the OLED has an optical transmission of at least about 50%. More preferably, the transparent cathode has optical transmission characteristics that permit the OLED to have an optical transmission of at least about 70%, still more preferably, at least about 85%. These requirements place significant limitations on the materials and thicknesses of the transparent cathode.
The transparent cathodes as disclosed in U.S. Pat. Nos. 5,703,436 and 5,707,745 typically comprise a thin layer of metal such as Mg:Ag with a thickness, for example, that is less than about 100 angstroms. The Mg:Ag layer is coated with a transparent, electrically-conductive, sputter-deposited, ITO layer. Such cathodes may be referred to as compound cathodes or as TOLED (xe2x80x9cTransparent-OLEDxe2x80x9d) cathodes. The thickness of the Mg:Ag and ITO layers in such compound cathodes may each be adjusted to produce the desired combination of both high optical transmission and high electrical conductivity, for example, an electrical conductivity as reflected by an overall cathode resistivity of about 30-100xcexa9/xe2x96xa1 (ohms per square). However, even though such a relatively low resistivity may be acceptable for certain types of applications, such a resistivity may still be somewhat too high for a passive matrix array of OLED pixels in which the current that powers each pixel needs to be conducted across the entire array through the narrow strips of the compound cathode.
It is known to use bus lines to mitigate limitations on the electrical conductivity of a transparent electrode. In the context of a passive matrix array of OLEDs, the bus line is a thick electrically conductive strip that runs parallel to a transparent electrode, and which provides electrical conductivity in the direction of the electrode. For example, U.S. Pat. No. 6,016,033 to Jones et al. discloses the use of a bus line in an array of OLEDs. Because the bus line is made of a thick electrically conductive material, it does not transmit light, and unfavorably results in an inactive area on the array of OLEDs. Because it is desirable to maximize the active area of an OLED display, it is desirable to minimize the area of the bus line. The active area may be quantified by a xe2x80x9cfill-factor,xe2x80x9d which is the percentage of the area of an array that is active or that emits light. Because of the enhanced electrical conductivity that is provided by a bus line, a bus line may be used notwithstanding the disadvantageous inactive area.
The organic materials of an OLED are very sensitive, and may be damaged by conventional semiconductor processing. For example, any exposure to high temperature or chemical processing may damage the organic layers and adversely affect device reliability. As a result, the processes conventionally used to fabricate a thick metal feature such as a bus line may damage any organic layers that are already present.
One technique that may be used to protect the delicate organic layers of an OLED is an integrated mask through which layers may be selectively deposited during fabrication. The mask is xe2x80x9cintegratedxe2x80x9d because it is left in place after fabrication, thus being integrated into the final device. Using an integrated mask is particularly desirable where the steps used to pattern material or to remove a mask have the potential to damage the device. Even where the integrated mask does not cover the delicate organic layers, the integrated mask protects the delicate organic layers by providing a patterning mechanism that does not require the patterning or removal of a mask once the organic layers are in place, i.e., the potentially damaging processes used to form the integrated mask are performed before the organic layers are present, and the potentially damaging processes used to remove a mask are not performed at all because the integrated mask is left in place. It is known to use an integrated mask to fabricate the top electrodes of an array of OLEDs, as disclosed in U.S. Pat. No. 5,701,055 to Nagayama et. al.
One problem that has been observed with conventional integrated masks is the shorting of adjacent electrode layers across the mask. It is known to use integrated masks having an overhang to mitigate shorting across the mask, as disclosed in U.S. Pat. No. 5,701,055 to Nagayama et. al. However, even with a conventional overhang, the process used to deposit the electrode must meet certain criteria to avoid shorting problems.
First, in order to avoid shorting problems, the xe2x80x9cfootprintxe2x80x9d of the deposited material, defined as the surface area onto which significant material is deposited, should be sharply limited to those surfaces having a direct line of sight, unobstructed by the mask, to the source of material being deposited. The footprint should not extend onto surfaces that do not have a direct line of sight to the source of deposited material. Whether deposition is limited in this way is dependent upon the deposition process. Processes such as low energy deposition of metals by thermal evaporation, generally result in footprint relatively sharply limited to surfaces having a direct line of sight to the source of material. In contrast, processes such as chemical vapor deposition may result in a significantly larger footprint, such that there is significant deposition onto surfaces not having a direct line of sight to the source of material. Processes such as sputter depositing have a footprint between that of thermal evaporation and that of chemical vapor deposition. One mechanism that may lead to a larger footprint is collisions between atoms or molecules of the material being deposited during transit to the substrate, and the resultant scattering. A low xe2x80x9csticking efficiencyxe2x80x9d of the atoms or molecules being deposited is another such mechanism. These mechanisms may lead to the side walls of a conventional integrated mask being coated with substantial quantities of the material being deposited even though such surfaces may not be within a direct line of sight from the source material.
Second, in order to avoid shorting problems, deposition that is significantly off-axis should be avoided, even for processes that have a limited footprint. Off-axis deposition is deposition from an angle not perpendicular to the substrate. Off-axis deposition may result in surfaces losing their protection from a direct line of sight to the source of material being deposited, such that material may be deposited into the recessed area under an overhang. Moreover, off-axis deposition often involves deposition from a variety of different angles, such that even more surface area loses protection from a direct line of sight during the process. Off-axis deposition may occur for many reasons. For example, the geometry of the substrate, the source, and their relative locations may lead to significant off-axis deposition, and may even lead to significant variations in the angle of deposition at different points on the substrate. The substrate may be placed on a moving conveyor belt during deposition, which inherently leads to off-axis deposition when the substrate is not directly beneath the source of material, and may lead to very large angle off-axis deposition when the substrate is at the edge of the deposition chamber. The substrate may also be rotated during deposition. Even processes that have a somewhat limited footprint, such as sputter depositing, may lead to shorting if there is significant off-axis deposition. It is believed that sputter depositing through a conventional integrated mask leads to shorting when off-axis deposition from angles of about 30 degrees or greater is present, although there may be shorting at smaller angles depending upon the exact process parameters.
It may be desirable, or even necessary in some cases, to use processes having a large footprint, and/or off-axis deposition, to fabricate certain types of layers that are used in an OLED. However, such processes may cause deposition to occur in undesirable regions of the device. For example, an ITO layer typically needs to be deposited using a high energy vacuum sputtering process that produces substantial scattering. Thus, using conventional integrated masks, it may not be possible to fabricate desirable but previously unattainable structures such as transparent or top-emitting OLEDs using a compound Mg:Ag/ITO cathode in a passive matrix OLED array. The use of such a conventional mask may result in the ITO layer causing harmful shorting across adjacent compound cathode strips.
FIG. 1 (prior art) illustrates the type of shorting of an electrode layer that may occur across an integrated mask due to the use of a deposition process that is not perfectly unidirectional. Mask 110 is fabricated on top of substrate 100. A metal layer 120 is deposited over mask 110, with the goal of fabricating electrodes 120a and 120b that are electrically separated. If the vapor deposition process is highly unidirectional, electrodes 120a and 120b, and residual layer 120d, are deposited. Any residual layer 120c that forms would not be continuous or thick enough to have significant electrical conductivity, because the vapor deposition process is highly unidirectional and there is no direct line of sight between the metal deposition source and residual layer 120c. However, if the vapor deposition process produces too much scattering and is, thus, insufficiently unidirectional, residual layer 120c may be continuous and have significant electrical conductivity. In this case, residual layers 120c and 120d may form a short between electrodes 120a and 120b. 
It would be desirable to be able to fabricate transparent or top-emitting OLEDs that exploit the high optical transmission of compound cathodes, such as Mg:Ag/ITO, in a passive matrix display, but without having such devices limited by the lower electrical conductivity of such compound cathodes. Furthermore, it would be desirable to be able to vapor deposit electrically conductive materials without encountering the shorting problems that may be experienced whenever such electrically conductive materials undergo substantial scattering during the deposition process.
In an embodiment of the invention, an organic light emitting device is provided. The device has a first electrode, an insulating strip disposed over a portion of the first electrode, and a bus line disposed on top of the insulating strip, such that the bus line is electrically insulated from the first electrode by the insulating strip. An integrated mask is disposed over the bus line, such that a portion of the bus line remains exposed vis-a-vis the insulating strip and the integrated mask. An organic layer is disposed over the first electrode, such that the organic layer is electrically connected to the first electrode. A second electrode is disposed over the organic layer, such that the second electrode is electrically connected to the organic layer, and such that the second electrode is electrically connected to the exposed portion of the bus line. A method of fabricating the device is also provided, which involves depositing the organic layer and the second electrode through the integrated mask, such that the second electrode is in electrical contact with the bus line.
The bus line may be disposed completely under the recessed area and completely under the overhang on one side of the base such that none of the bus line contributes to a loss in the fill factor of the OLED array. The recessed area on at least one side of the base has an aspect ratio sufficiently large such that no vapor deposition may occur in the furthermost interior, distal, depths of the recessed area even when highly scattered materials are vapor deposited. However, the outermost, proximal, exposed portion of the bus line is sufficiently close to the outermost extension of the overhang so as to allow vapor deposition of a highly scattered second electrode material to make electrical contact with at least a proximal portion of the exposed portion of the bus line.
In an embodiment of the invention, a photoresist mask is provided, having a central region fabricated on an underlying layer. An overhang supported by the central region is separated from the underlying layer by a recessed area. The recessed area has an aspect ratio of at least about 1.5. The mask may be advantageously used to pattern electrodes deposited through the mask by chemical vapor deposition or sputtering, such that there is no significant conductivity across the mask between the patterned electrodes.